The present invention relates to a process for producing methacrylic acid by the vapor phase catalytic oxidation of methacrolein with the molecular oxygen or a molecular oxygen-containing gas. More particularly, it relates to a process for producing methacrylic acid in high yield from methacrolein by using a novel multi-elemental catalyst of complex oxide containing Mo, P, As, V, Cu, Ce and O, if necessary, Cr with or without a reducing organic agent.
Many patents and patent applications on various processes for producing unsaturated carboxylic acids such as acrylic acid or methacrylic acid by the vapor phase catalytic oxidation of the unsaturated aldehyde such as acrolein or methacrolein have been disclosed.
The process for producing acrylic acid by the oxidation of acrolein have been practically worked in a large industrial size because of development of the catalytic systems having high catalytic activity.
However, in the process for producing methacrylic acid from methacrolein which is similar to the former, the result has not been satisfactory for the industrial operation and it has not been practically applied.
Various catalysts have been proposed as the catalysts for producing acrylic acid from acrolein.
In usual, the Mo-V type catalysts have high catalytic activities. In Japanese Patent Publication Nos. 1775/1966, 12129/1969 and 19296/1973, high single flow yields of acrylic acid such as about 80% have been obtained.
However, even though the oxidation of methacrolein was carried out by using these known Mo-V type catalysts, the amounts of the by-products of carbon monoxide, carbon dioxide and acetic acid have been high whereby the single flow yield of methacrylic acid has been remarkably low. The reason is that methacrylic acid is easily and gradually decomposed by the oxidation of the catalysts.
As the catalysts for producing methacrylic acid by the vapor phase oxidation of methacrolein, the heterpolyacid type catalysts such as Mo-P type catalysts which result in mild reaction have been mainly developed. Many patent applications have been filed on these type catalysts such as the catalysts comprising the components of Mo, P, Tl and at least one of Si, Cr, Al and Ti in Japanese Patent Publication No. 24288/1975; the catalysts comprising the components of Mo, P, V and at least one of Na, K, Rb and Cs in Japanese Unexamined Patent Publication No. 96522/1975; the catalysts comprising the components of Mo, P, V and at least one of K, Rb, Cs and Ta or further comprising the component of at least one of Sr, Zn, Cd, Nb, B, Pb, Bi and W in Japanese Unexamined Patent Publication No. 123619/1975; the catalysts comprising the components of Mo, P, V and Sb, or further comprising the component of at least one of Fe, Ni, Mg, Ca, Al, W, Cu, Zn and Cr in Japanese Unexamined Patent Publication No. 23216/1976.
However, the known Mo-P type catalysts usually have low catalytic activity and require high reaction temperature.
As described in Japanese Patent Publication No. 27526/1965 and Japanese Unexamined Patent Publication No. 33082/1972, remarkable deterioration of catalytic activity has been caused by the thermal deterioration etc. and a satisfactory catalytic life could not be obtained disadvantageously in the industrial operation.
In the industrial operation, it is usually important to use the catalyst which imparts high conversion of methacrolein fed and high selectivity of methacrylic acid and it is also important and indispensable to give high catalytic activity (space time yield) and high durability of catalyst (life). From these viewpoints, the conventional process for producing methacrylic acid has not been satisfactory in the industrial operation.
The inventors have proposed the catalysts comprising the components of Mo, P, V, Ce and Cu as the catalyst for producing methacrylic acid from methacrolein. These catalysts impart relatively high catalytic activity at low temperature and high durability of the catalysts, however the satisfactory selectivity of methacrylic acid could not be attained by these catalysts.
The inventors have further studied to improve the selectivity by adding various elements to the catalyst system. As the results, the inventors have found that the selectivity could be remarkably improved by adding As component to the components of Mo, P, V, Ce and Cu. The present invention has been attained by the finding.
On the other hand, as the catalysts comprising As component which relate to the present invention, the catalysts comprising the components of Mo, P and As have been proposed in Japanese Patent Publication Nos. 10652/1974 and 3297/1975.
When these catalysts are used, remarkably high selectivity could be attained. However, the catalytic activity is remarkably low and the catalytic life is not always satisfactory.
Accordingly, catalysts have been proposed which further comprise an additional metal element or both the metal element and alkali metal element such as the catalysts comprising the components of Mo, P and As and one or two of V, Cu, Fe and Mn in Japanese Patent Publication No. 23014/1975; the catalysts comprising the components of Mo, P, As and at least one of V, W, Cu, Fe, Mn and Sn or further the component of at least one of Li, Na, K, Rb and Cs in Japanese Unexamined Patent Publication No. 41811/1975; the catalysts comprising the components of Mo, P, As, Cu or V and at least one of Li, Na, K, Rb and Cs and at least one of Mg, Al, Si, Ca, Ti, Zr, Ag, Sb, Te, Ba and Ta in Japanese Unexamined Patent Publication No. 115414/1976.
Certain catalysts impart relatively high single flow yield of methacrylic acid, however, the catalytic activity at relatively low temperature of about 300.degree. C. is not satisfactorily high.
The catalysts of the present invention impart excellent catalytic activity and durability in comparison with these known catalysts. The reason is considered to be given by the effect of Ce.
As the catalysts comprising Ce component which relate to the present invention, catalysts comprising the components of Mo, P and V or the addition of at least one of Bi, As, B, Ce Cr, Ag, Fe, W, Pb, Mn, Ta, Te, Ni, Nb, Sn and Cu have been proposed in U.S. Pat. No. 3,875,220; the catalysts comprising the components of Mo, P, V and W or the addition of at least one of Sn, Sb, As, Cu, Ce, B, Cr, Fe, Ni, Co, U, Mn, Ag, Pb, Rh, Cd, Bi, In, Zn and La in Japanese Unexamined Patent Publication No. 115416/1976; the catalysts comprising the components of Mo, P and at least one of K, Rb, Cs and Tl, and at least one of Rh, Ce and Zr in Japanese Unexamined Patent Publication No. 108016/1976, etc.
The inventors have proposed the catalysts comprising the components of Mo, P, V and Ce in Japanese Unexamined Patent Publication No. 136615/1976. However, these catalysts have low catalytic activity and could not impart satisfactory yield of methacrylic acid and the catalytic life in the industrial operation.
The inventors have further studied to find the improved catalysts. As the results, the inventors have found that remarkably excellent catalytic characteristics could be obtained by adding Cr component to the components of Mo, P, As, V, Cu and Ce.
Moreover, the inventors have found that when an organic compound such as dibasic carboxylic acids, oxycarboxylic acids is added for the reduction during the preparation of the catalyst, the yield is increased and the satisfactory results can be obtained in the industrial operation.